1. Field of the Invention
The present invention relates to a method of making a silicon semiconductor film having crystalline performance, for example, a polycrystal silicon film, a single crystal silicon film or a microcrystal silicon film. A crystalline silicon film fabricated by using the present invention is used in various semiconductor devices.
2. Description of the Related Art
There has been known a thin film transistor (hereinafter, referred to as TFT or the like) using a thin film semiconductor. TFT is constituted by forming a thin film semiconductor, particularly a silicon semiconductor film on a substrate and by using the thin film semiconductor. Although TFT is utilized in various integrated circuits, TFT attracts attention particularly as a switching element provided to each pixel of an active matrix type liquid crystal display device and a driver element formed at a peripheral circuit portion thereof. Further, TFT attracts attention as a technology indispensable also in a multiple structure integrated circuit (three-dimensional IC).
Although it is convenient to use an amorphous silicon film for a silicon film utilized in TFT, electric properties thereof are far lower than those of a single crystal semiconductor film used in a semiconductor integrated circuit. Accordingly, TFT has been used only in a limited usage such as a switching element of an active matrix circuit. A silicon thin film having crystalline performance may be utilized for promoting the properties of TFT.
A silicon film having crystalline performance is referred to as a polycrystal silicon film, a polysilicon film, a microcrystal silicon film or the like other than a single crystal silicon film. To provide such a silicon film having crystalline performance, an amorphous silicon film is firstly formed and thereafter, the film is crystallized by heating (thermally annealing) the film. The process is referred to as solid phase growth process since an amorphous state is transformed into a crystalline state while maintaining a solid state.
However, in the solid state growth of silicon, a heating temperature of 600xc2x0 C. or higher and a time period of 10 hours or more are needed and an inexpensive glass substrate is difficult to use as a substrate. For example, Corning 7059 glass used in an active type liquid crystal display device is provided with the strain point of glass of 593xc2x0 C. which is problematic in performing thermal annealing at 600xc2x0 C. or higher when large area formation of the substrate is considered.
According to a research conducted by the inventors on such a problem, it has been found that when a certain kind of metal element of nickel or the like is piled up on the surface of an amorphous silicon film by a small amount and thereafter the film is heated, the crystallization can be performed at 550xc2x0 C. for a processing time period of about 4 hours. Naturally, a silicon film having a further excellent crystalline performance can be provided by performing the annealing process at 600xc2x0 C. for 4 hours. (refer to Japanese Unexamined Patent Publication No. JP-A-6-244103).
In order to introduce such a small amount of metal element, there are a method of piling up a film of a catalyst element or a compound thereof by a sputtering process (Japanese Unexamined Patent Publication No. JP-A-6-244104), a method of forming a film of a metal element or a compound thereof by means of spin coating (Japanese Unexamined Patent Publication No. JP-A-7-130652), a method of forming a film by decomposing a gas including a metal element by thermal decomposition, plasma decomposition or the like (Japanese Unexamined Patent Publication No. JP-A-7-335548) and the like which may be used according to the respective characteristic.
Further, it is possible to introduce a metal element selectively at a specific portion and to expand crystal growth from a portion where the metal element is introduced to surroundings by heating (lateral growth process or horizontal growth process). Crystal silicon provided by such a method has an orientation of crystallization and therefore, extremely excellent properties are shown in accordance with the orientation.
Further, it is also effective to further improve crystalline performance by irradiating strong beam such as laser beam or the like after a crystallizing step using a metal element (Japanese Unexamined Patent Publication No. JP-A-7-307286). Further, according to the above-described lateral growth process, it is effective to perform thermal oxidation in succession thereto (Japanese Unexamined Patent Publication No. JP-A-7-66425).
When crystallization is performed by using a metal element in this way, a crystalline silicon film having excellent quality is provided at a lower temperature in a shorter period of time. A temperature of heating treatment is preferably falls in a range of 450 through 650xc2x0 C., particularly preferably, in a range of 550 through 600xc2x0 C., although strongly dependent on the kind of an amorphous silicon film.
However, the most serious problem in the method is removal of metal element. It cannot be disregarded that a metal element introduced in a silicon film effects adverse influence on the electric properties and reliability. Particularly, in the step of crystallization using a metal element, as the mechanism of step, the metal element remains in the film mainly as a conductive silicide which constitutes a significant factor of defect.
It is generally known that a metal element (particularly, nickel, palladium, platinum, copper, silver or gold) can be removed by a heating treatment in an atmosphere of hydrogen chloride at high temperatures. However, a high temperature treatment at about 1000xc2x0 C. is needed therefor which is against the thought of low temperature process using a metal element. The present invention has been carried out in view of the above-described problem and it is an object of the present invention to provide a method effective in removing a metal element by providing conditions preferable for gettering.
According to the invention disclosed in the specification, a region implanted with an element of 15 group (representatively, phosphorus) at high concentration is firstly provided in a silicon film, contiguous to a region intended to remove a metal element initially.
This region is impaired by implantation of the element of 15 group. Further, by performing a heating treatment, a metal element for promoting crystallization is made to move to the region to which the element of 15 group is acceleratingly implanted.
With respect to the region acceleratingly implanted with the element of 15 group:
(1) High density of unpaired bonds are formed by implanting ions.
(2) The element of 15 groups per se is provided with a property of bonding with a metal element (the property is particularly significant in phosphorus).
Accordingly, the movement of a metal element in accordance with a heating treatment to a region implanted with an element of 15 group as described above, is irreversible.
Therefore, by performing a heating treatment, as a result, a state where a metal element for promoting crystallization is moved from a region where the element of 15 group is implanted to a region where the 15 group is not implanted, is provided.
Particularly, when phosphorus is utilized, the above-described operation can significantly be obtained since phosphorus and nickel constitutes a stable bondage state at a temperature around 600xc2x0 C.
Phosphorus and nickel is provided with a number of bondage states such as Ni3P, Ni5P2, Ni2P, Ni3P2, Ni2P3, NiP2, NiP3.
Therefore, when nickel is adopted as a metal element promoting crystallization and phosphorus is adopted as an element of 15 group, nickel can be drawn as an object of bondage with phosphorus very effectively. That is, gettering can be performed effectively.
Photographs shown by FIGS. 7A and 7B show an effect of gettering. The photograph shown by FIG. 7A indicates a pattern of a silicon film in which gettering is performed to a crystalline silicon film obtained by utilizing nickel in accordance with a first aspect of the present invention.
In the state shown by FIG. 7A, nickel has been absorbed to a region outside of the pattern (region is removed in photograph).
FIG. 7B shows a pattern of a silicon film when gettering of nickel is not performed. Speckles observed in the pattern of the silicon film shown by FIG. 7B are fine openings showing a state where remaining nickel and nickel compounds have been removed. This state is obtained by performing a treatment by using a special etchant (mixture of hydrofluoric acid, hydrogen peroxide and water) capable of removing nickel and nickel compounds with high selectivity.
Although the treatment by using the above-described etchant is performed in respect of the pattern shown by FIG. 7A, the speckles shown by FIG. 7B are not observed since nickel has been removed by a gettering step and the nickel compound is not present in the pattern.
Further, FIG. 11A shows the characteristic of TFT fabricated by using a film in correspondence with FIG. 7A, and FIG. 11B shows the characteristic of TFT fabricated by using a film in correspondence with FIG. 7B. Further, in fabricating TFTs, the treatment of an etchant for removing nickel is not performed.
As is apparent from FIGS. 11A and 11B, when nickel remains in an activation layer, the OFF characteristic is significantly deteriorated. Further, although a representative characteristic is shown in FIGS. 11A and 11B, when the film shown by FIG. 7B is used, a dispersion of the characteristic per se is significantly large.
According to a first aspect of the present invention disclosed in the specification there is provided a method of making a semiconductor device including;
a step of forming a crystalline film by crystallizing an amorphous silicon film or an amorphous film including silicon by using a metal promoting to crystallize silicon,
a step of accumulating stress and strain by irradiating a pulse laser beam or an equivalent strong beam to the crystalline film,
a step of selectively forming a mask on the crystalline film,
a step of adding an element selected from 15 group to the crystalline film by using the mask, and
a step of performing a heating treatment and gettering the metal from a region where the element is not added to a region where the element is implanted.
According to a second aspect of the present invention, there is provided a method of making a semiconductor device including;
a step of forming a crystalline film by crystallizing an amorphous silicon film or an amorphous film including silicon by using a metal promoting to crystallize silicon,
a step of selectively forming a mask on the crystalline film,
a step of accumulating stress and strain in a region other than a region where the mask is formed by irradiating a pulse laser beam or an equivalent strong beam to the crystalline film,
a step of accelerating and implanting an element selected from 15 group to the crystalline film by using the mask to thereby impair a region implanted with the element, and
a step of performing a heating treatment and gettering the metal from a region where the element is not implanted to a region where the element is implanted.
An amorphous silicon film is generally used as an amorphous film. However, a compound of silicon and other element, for example, a compound semiconductor designated-by Six Ge1xe2x88x92x (0 less than x less than 1 ) can be used. Further, a film added with impurities can be used for controlling film quality or for controlling the electric properties of a device. For example, an amorphous silicon film having one conductive type or the like can be utilized.
A single or a plurality of kinds of elements selected from the group consisting of Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir, Pt and Au can be used as the metal element.
It is particularly preferable to use Ni (nickel) in view of reproducibility and effect.
Further, in gettering, grain boundaries in a silicon film constitute a hazard in moving the metal element. Generally in a silicon film immediately after solid phase growth, metal elements are precipitated as silicides at grain boundaries and as a result, the grain boundaries grow, however, such silicides are thermodynamically stable (in the first place, metal elements are precipitated at grain boundaries since the precipitation is thermodynamically stable) and the metal elements are difficult to move from these portions. Furthermore, metal elements moved from other portions are caught and fixed thereby.
In contrast thereto, when laser annealing treatment is performed by irradiating pulse laser beam to a silicon film which has been crystallized by solid phase growth, remaining amorphous components are crystallized and further, the tendency of precipitating the metal elements at grain boundaries is significantly reduced.
The tendency of precipitating the metal elements is reduced because the thermodynamic state is accompanied by an abrupt change when a pulse laser (particularly having a pulse width of 1 xcexcsec or lower) is irradiated and growth of crystal grains and formation of grain boundaries are not progressed sufficiently. (On the other hand, crystallization is progressed in respect of remaining amorphous components.)
The state of being irradiated with a pulse laser beam may be regarded as the state where stress and strain is accumulated in the silicon film. In that state, many metal elements are present in the silicon film by being dispersed between lattices of silicon and therefore, the metal elements are very easy to move. Further, there are few large grain boundaries catching the metal elements and therefore, later gettering can be performed efficiently.
The operation and effect can significantly be obtained by light irradiation of a pulse oscillation type, preferably irradiation of a laser beam of a pulse oscillation type compared with simple light irradiation.
According to the present invention disclosed in the specification, it is preferable that the concentration of the element of 15 group which is acceleratingly implanted, is higher than the concentration of the metal element for promoting crystallization by one digit or more.
For example, the concentration is preferably as high as 5xc3x971019 through 2xc3x971021 atoms/cm3.
Further, it is effective in view of a total of the film that a total amount of implanted phosphorus element is made larger than a total amount of nickel element remaining in the film, more preferably, a total amount of implanted phosphorus element is made larger than a total amount of nickel element remaining in the film by 10 times or higher.
Further, in accelerating and implanting of the element of 15 group, an element such as hydrogen, oxygen or carbon may simultaneously be implanted at a concentration of 1xc3x971019 through 1xc3x971021 atoms/cm3. When these elements are present in a large amount, crystallization of the film in heating treatment for moving the metal elements can be hampered.
It is important for promoting the effect of gettering to form unpaired bonds at high density in a region where phosphorus is implanted and for that purpose, the above-described device of hampering crystallization becomes useful.
According to the present invention, gettering is carried out in a step of forming to partition an activation layer of a transistor by etching a silicon film. Although a portion of the region where the element of 15 group is implanted may totally be removed, the portion may be used as a portion or a total of a source or a drain of a transistor. When the region is used as a portion or a total of a source or a drain of a P-channel type transistor, a P type region may be formed by implanting a P type or an N type impurity exceeding an amount of implanting the element of 15 group.